How do we keep the ultrasonic wand in place on a stomach?

The head of the wand needs to contact the skin. I'm not sure the wand has to be at the spot you want to help though. The body fluids allow the ultrasound to travel around easily. Putting the wand under the armpit may be quite adequate.

The ultrasonic wand could also be placed at the bottom of the abdomen, between the legs and secured by an Ace bandage - which would place the frequency closer to his issue.